An external floating roof tank is a

storage tank Storage tanks are containers that hold liquids, compressed gases (gas tank; or in U.S.A "pressure vessel", which is not typically labeled or regulated as a storage tank) or mediums used for the short- or long-term storage of heat or cold. The t ...

commonly used to store large quantities of petroleum products such as crude oil or condensate. It consists of an open- topped cylindrical steel shell equipped with a roof that floats on the surface of the stored liquid. The roof rises and falls with the liquid level in the tank. As opposed to a

fixed roof tank A fixed roof tank is a type of storage tank, used to store liquids, consisting of a cone- or dome-shaped roof that is permanently affixed to a cylindrical shell. Newer storage tanks are typically fully welded and designed to be both liquid- and vapo ...

there is no vapor space (

ullage Ullage or headspace is the unfilled space in a container, particularly with a liquid. Etymology The word ''ullage'' comes from the Latin word ''oculus'', used by the Romans to refer to a barrel cork hole. This word was, in turn, taken in medieval ...

) in the floating roof tank (except for very low liquid level situations). In principle, this eliminates tank breathing loss and greatly reduces the evaporative loss of the stored liquid. There is a rim seal system between the tank shell and roof to reduce rim evaporation. The roof has support legs hanging down into the liquid. At low liquid levels the roof eventually lands and a vapor space forms between the liquid surface and the roof, similar to a

. The support legs are usually retractable to increase the working volume of the tank.

Advantages

External roof tanks are usually installed for environmental or economical reasons to limit product loss and reduce the likelihood of emission of

volatile organic compounds Volatile organic compounds (VOCs) are organic compounds that have a high vapour pressure at room temperature Colloquially, "room temperature" is a range of air temperatures that most people prefer for indoor settings. It feels comfortable to a ...

(VOCs) and other potential

air pollutant Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different types ...

s. Normally, the roof of the tank rests directly on the surface of the stored liquid, leaving little vapor space and consequently leading to a much lower risk of rim space fire.

Disadvantages

Snow and rain water can accumulate on the roof; the roofs are designed to hold up to a 24 hour 10" / 255mm rain event. Roofs are exposed to sunlight and weather which can accelerate corrosion.

System function

Water on the roof is usually drained by a special flexible hose or other special drain line system that runs from drain-sumps on the roof, through the stored liquid to a drain valve on the shell at the base of the tank. A hose is the shortest quickest route, Other drain systems are available both rigid and semi-rigid. These are named 'Articulated' as they use straight lengths of steel pipes with mechanical swivel joints or consist of steel pipes with flexible sections.

References

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External links

Use of floating roof storage tanks in Oil & Gas industry

Compilation of Air Pollutant Emission Factors, Volume 1: Stationary Point and Area Sources. Chapter 7. Chapter 7: Liquid Storage Tanks
Oil storage Petroleum production Storage tanks

Advantages

Disadvantages

System function

See also

References

External links